Fiber-reinforced polymer-matrix composite materials are used for a variety of structural applications. These composite materials, which include a matrix constituent, typically a resin, and a reinforcement constituent, typically a fiber bundle or woven fabric, are formed via a molding operation.
One method for forming composite materials is a process called “resin transfer molding” or “RTM.” In this process, resin is added under pressure into a closed-cavity mold. In the simplest version of RTM, air is left in the fibers before resin injection. Some but not all of this air is driven out through vents as the fibers fill with resin. In order to obtain an acceptable void content in the presence of this residual air, a very high pressure (about 275 psig) is sometimes applied while the resin is curing. The intent of the applied pressure is to shrink the size of any remaining air voids to acceptable levels. This large internal pressure generates substantial forces that tend to push opposing mold surfaces apart. For small molds, this problem is addressed using relatively inexpensive presses. But this approach becomes impractically expensive when dealing with large molds.
Another liquid-resin process is vacuum-assisted resin transfer molding (“VARTM”). In this process, air is driven out of the reinforcement constituent by placing it under vacuum conditions. FIG. 1 depicts a simplified representation of conventional, horizontally oriented, VARTM molding apparatus 100.
As depicted in FIG. 1, apparatus 100 includes hard tool 102 (i.e., the mold) and soft tool 112, which is traditionally implemented as a flexible membrane, such as nylon vacuum bagging film, a sheet of silicon rubber, or similar material. The term “tooling” or “tool” refers to a solid entity/surface against which the composite material is molded; it forms the shape of the molded article (“workpiece”) as the liquid resin transforms into a solid. The soft tool is sealed to the hard tool or another appropriate surface to create or gas-tight chamber 114. During operation, air is evacuated from chamber 114; for this reason, the membrane is sometimes referred to as a “vacuum bag.”
Resin is introduced into chamber 114 to impregnate the reinforcement component, typically fibers/fabric 108 (hereinafter “fiber preform 108”), which is already arranged in that region. Also disposed in chamber 114 are mold release (applied as a liquid or solid) film 104, peel ply 106, resin distribution medium 110, resin distribution line(s) 117, and vacuum distribution line(s) 118. The peel ply and release film provide a releasing interface to make it easier to separate various layers (e.g., resin distribution medium, etc.) from the finished workpiece. Resin distribution medium 110 is an open-structured coarse medium used initially as a vacuum pathway to evacuate air from dry fiber preform prior to resin infusion. As its name implies, resin distribution medium 110 is primarily used to rapidly and evenly distribute resin to fiber preform 108.
Fiber preform 108 is thus sandwiched between hard tool 102 and soft tool 112 in chamber 114. In operation, a vacuum is pulled in chamber 114 via vacuum line 118, thereby drawing soft tool 112 against resin distribution medium 110. The pressure differential across the soft tool (atmospheric pressure on one side, vacuum on the other) results in a compaction pressure that compacts the fiber preform. This is required to obtain a composite with a controlled and desired fiber volume fraction. Resin is introduced through soft tool 112 via one or more resin inlet lines 116. Resin is fed from resin inlet line(s) 116 to resin distribution line(s) 117. The resin distribution line(s) provides a way to distribute resin across resin distribution medium 110. The resin rapidly penetrates along the resin distribution medium, is infused throughout the fiber perform, and is then cured.
In conventional VARTM systems, the resin inlet line(s) 116 and resin distribution line(s) 117 remain filled with resin. The resin in these lines is allowed to cure along with the curing workpiece. Typically, the lines are discarded after the workpiece is removed from the mold and new resin inlet lines and resin distribution lines are installed for each subsequent VARTM molding operation. As such, these lines are effectively single-use lines.
It is time consuming to replace the resin inlet and distribution lines for each molding run. And in applications in which the VARTM process is used repeatedly on the same geometry, replacing the lines seems particularly inefficient and costly.
The prior-art has addressed the problem of single-use resin distribution lines with the development of the temporary or reusable resin distribution line. Several different configurations/approaches for a re-usable resin distribution line have been proposed.
One approach, which is depicted in FIGS. 2A through 2D, is a process called “FASTRAC.” Like conventional VARTM, FASTRAC apparatus 200 includes hard tool 202 and soft tool 212, with fiber preform 208 sandwiched therebetween. Primary vacuum line 218 is operable to evacuate air from first gas-tight chamber 214 “under” soft tool 212.
Referring now to FIG. 2A and the “magnified” view of FIG. 2B, unlike a conventional VARTM process, FASTRAC apparatus 200 also includes FASTRAC layer 220. This layer is a semi-rigid support layer that has a plurality of channels 224 formed in surface 222. FASTRAC layer 220 is disposed “outside” of soft tool 212. The FASTRAC layer is sealed to the hard tool or other appropriate surface to create a second gas-tight chamber 226 above soft tool 212. Secondary vacuum line 228 is operable to evacuate air from this second gas-tight region. FASTRAC apparatus 200 thus includes two “vacuum bags”—one created by soft tool 212 and the second via FASTRAC layer 220.
Referring now to FIG. 2C and the “magnified” view of FIG. 2D, in operation, a vacuum is first drawn in second chamber 226. This deforms soft tool 212 into the channels 224 of FASTRAC layer 220. This operation creates channels 230 through which resin flows and distributes across the top of fiber preform 208. After channels 230 are formed, the pressure in first chamber 214 is reduced to evacuate air, as per standard VARTM processing. Atmospheric pressure holds FASTRAC layer 220 against fiber preform 208 while, at the same time, channels 230 maintain their shape due to the semi-rigid FASTRAC layer. Resin is then injected into the first chamber (e.g., via resin inlet line 216). The resin flows according to the geometry of channels 230 in FASTRAC layer 220. After the appropriate amount of resin is injected, vacuum in the second chamber is released such that the second chamber is then under atmospheric pressure. The atmospheric pressure in the second chamber causes channels 230 to collapse, forcing soft tool 212 against fiber preform 208. This forces resin into the fiber preform.
The FASTRAC process therefore avoids the use of conventional single-use resin distribution line(s) as well as resin distribution medium by forming channels 230. But the process is complex; accurate control of the sub-atmospheric pressure levels in first chamber 214 and second chamber 226 is critical and timing issues related to resin injection rates and re-pressurizing the second chamber are also very important.
A second approach to creating a reusable resin distribution line is presented in Publ. Pat. Appl. US 2007/0063393. This reference discloses a VARTM process wherein a flow channel is created on the top face of the fiber preform to accelerate resin flow and reduce resin injection time. According to the reference, the channel is created by lifting the soft tool via a pressure differential.
With reference to FIG. 3, which depicts the apparatus disclosed in US 2007/0063393, apparatus 300 includes hard tool 302 and soft tool 312, with fiber preform 308 disposed therebetween. Primary vacuum line 318 is operable to evacuate air from fiber perform 308 “under” soft tool 312. Unique to this process, hard shell 320 is disposed over soft tool 312. Secondary vacuum line 328 is capable of drawing a vacuum in region 326 between shell 320 and soft tool 312. Resin inlet line 316 delivers resin to high permeability channel 332.
In operation, pressure is reduced beneath soft tool 312 via primary vacuum line 318. This draws the soft tool against fiber preform 308, which compacts the fiber preform. While the region beneath soft tool 312 is maintained under reduced pressure, region 326 above the soft tool is reduced to an even lower pressure via secondary vacuum line 328. This causes soft tool 312 to stretch away from the top of fiber preform 308 creating flow channel 314, as depicted in FIG. 3.
Resin is delivered, through resin inlet line(s) 316, to high-permeability region 332. Resin preferentially flows from the high-permeabilty region to flow channel 314 since fiber preform 308 presents a much greater resistance to flow. After the requisite amount of resin is delivered to flow channel 314, region 326 above soft tool 312 is pressurized to atmospheric pressure. The positive pressure in region 326 compresses soft tool 312 against fiber preform 308, which drives the resin into the fiber preform.
The process and apparatus disclosed in US 2007/0063393 avoids the use of conventional single-use resin distribution line(s) and resin distribution medium by forming flow channel 314. The approach taken is, however, quite problematic in terms of workpiece quality (freedom from voids and dry spots), fiber volume fraction, and complexity. As to workpiece quality, the vacuum above soft tool 312 must be stronger than the vacuum under the soft tool (to lift the soft tool to create the flow channel). Since, by definition, a vacuum greater than 1 atmosphere cannot be created, there will necessarily be some partial pressure of air beneath the soft tool in fiber preform 308. For VARTM, the highest workpiece quality (i.e., lowest void content) results when using the highest vacuum possible within the fiber preform. This leaves a minimum amount of residual air in the fiber preform. Residual air can, especially if trapped by complex geometry, lead to voids in the finished part. In extreme cases, the residual air can collect in “dead regions” (regions unconnected to the vacuum port), which can result in “dry” (resin-less) regions in the fiber preform.
US 2007/0063393 even discloses that the fiber volume fraction will be low because of the lack of compaction on the fiber preform. Some mitigation measures are discussed, but ultimately, fiber volume fraction may, in many cases, be lower than desired. Furthermore, the shell (i.e., FIG. 3: shell 320) that is required must support internal vacuum; in other words, a net external pressure of about 14.7 psi. When manufacturing large composite parts, this pressure-supporting shell will necessarily be quite massive and expensive. And, for complex-shaped parts, forming the shell, especially a large one, can be very difficult.
The art would therefore benefit from a molding process that addresses the problem of single-use resin distribution lines but avoids the drawbacks of the solutions heretofore presented.